Electrolyte migration during fuel cell stack operation is a persistent problem, and can result in cells drying out at the lower (positive) end of the stacks, and cells flooding at the upper (negative) end of the stack. U.S. Pat. No. 4,761,348 granted Aug. 2, 1988 to H. R. Kunz, et al. addresses the problem. This patent offers a solution which entails providing inactive electrolyte reservoirs at each end of the stack, with the negative end reservoir being initially low in electrolyte; and the positive end reservoir being initial high in electrolyte fill. This patent also describes the use of high porosity, low electrolyte retention seals which will limit electrolyte migration during stack operation. The patent suggests the use of lithium aluminate fibers to form a highly porous fibrous ceramic strip which will be capable of containing no more than 4% by volume of the molten electrolyte. The fibers are said to be smooth and have a diameter of about 0.5 to 20 microns. The problem with using lithium aluminate fibers to form a sealing mat is that the fibers are so short as to be unable to form a seal mat that is stable over time during stack operation and the mat does not possess the necessary shear strength for vertical seals in the fuel cell stack. These short fibers cannot be effectively cast or felted into a stable seal mat. The use of lithium aluminate fibers is thus not a satisfactory solution to the problem of electrolyte migration in a molten carbonate fuel cell stack.
U.S. Pat. No. 4,548,874 granted Oct. 22, 1985 to M. Katz, et al. suggests the use of lithium aluminate for use in a porous sealing gasket. This patent suggests that the sealing gasket could be formed from a lithium aluminate mat. The same problems and drawbacks found in forming a sealing gasket mat of lithium aluminate fibers which are inherent in the Kunz, et al. reference, are also present in the Katz, et al. teachings. Thus the Katz, et al. reference does not offer a viable solution for lithium carbonate electrolyte migration during extended stack operation.
Another attempt to control electrolyte migration in a molten carbonate fuel cell stack is found in U.S. Pat. No. 4,659,635, granted Apr. 23, 1987 to C. A. Reiser, et al. This patent describes a solution which involves forming marginal portions of the electrolyte matrices with larger pore sizes than the central portions of the matrices. This solution attempts to prevent the electrolyte from migrating to the outer edges of the matrices so that it cannot reach the seals. The difficulty with this solution relates to the forming of a matrix plate with one portion having a low porosity so that it can retain the electrolyte because of high capillary forces, and another portion with a high porosity so that it cannot retain electrolyte because of low capillary forces.